1. Field of the Invention
The present invention relates to a rotation detecting apparatus for measuring a rotation speed, a rotational displacement and so on of a rotating object and a scale for rotation detection.
2. Related Background Art
There are conventional apparatus utilized for obtaining a physical amount such as a rotation angle or an angular velocity of an object at high accuracy while irradiating light onto the object, for example optical rotary encoders. These apparatus utilizing light are featured by their high accuracy and high resolution, but are desired to have a smaller size (i.e., a size on the millimeter order), a higher accuracy, a higher resolution (of 0.1 .mu.m order), and higher stability in order to be applied in wider fields. A millimeter size apparatus could be used as directly bonded to a measured object, and therefore can be used for smaller apparatus.
FIG. 1A is a perspective view to show an example of a conventional optical rotary encoder as described for example in Japanese Laid-open Patent Application No. 5-157583. In FIG. 1A, reference numeral 101 denotes a light source such as a semiconductor laser, 102 a lens, 103 a beam splitter, 104 a rotary disk plate on which a diffraction grating 105 is formed radially on a rotational trace of a position where a beam passing through the beam splitter 103 is incident at right angles and at constant intervals, 106 a condenser lens for condensing beams reflected by the diffraction grating 105 and then also reflected by the beam splitter 103, 107 a beam splitter for reflecting the beams from the condenser lens 106 to 10 make the beams incident at right angles onto the diffraction grating 105 on the rotary disk plate 104, 108 a mirror for reflecting beams reflected by the diffraction grating 105 and then passing through the beam splitter 107, and 109 a beam splitter for guiding the beams reflected by the mirror 108 to a light-receiving element 110.
The conventional optical rotary encoders have been utilizing a disk plate 6a on which radial gratings are formed as shown in FIG. 1B as light phase modulating means used as a scale. Since such a rotary encoder of the above structure is so arranged that the diffraction grating as the light phase modulating means is radially formed on the disk plate, the grating pitch differs in the radial direction. This pitch change could be a cause of measurement error. Then an arrangement is necessary for precisely setting a grating reading position of a reading head in the radial direction or for correcting a change in the position when caused. It was thus difficult to achieve both a size reduction and an increase of resolution at the same time.
Also, such apparatus could be constructed only in the arrangement for detecting a circumferential displacement of the grating, and the location of the head unit was limited to one for detecting the circumferential displacement, which made the size reduction difficult as well.